The blood pressure is one of barometers to analyze circulatory diseases, and performing risk analysis based on the blood pressure is effective for prevention of cardiovascular diseases such as stroke, heart failure and cardiac infarction. Among these, early-morning high blood pressure, in which the blood pressure rises in the early morning, is linked to cardiac diseases, stroke, and the like. Furthermore, in the early-morning high blood pressure, it has been found that a symptom of rapid increase in the blood pressure in one to one and a half hours after awakening, which is called morning surge, has a cause-and-effect relationship with stroke.
Thus, there have been proposed various sphygmomanometers capable of automatically measuring a systolic blood pressure and a diastolic blood pressure.
For example, electronic sphygmomanometers for measuring the blood pressure by the oscillometric method (oscillation method) have been marketed. In such electronic sphygmomanometers, in a process in which a pressure inside an arm band (cuff) wound around a measurement site (cuff pressure) is increased to become higher than a systolic blood pressure, and then the cuff pressure is gradually decreased, volume change of an artery accompanying pulsation of the blood pressure is detected by a pressure sensor as oscillation of the cuff pressure (e.g., Japanese Examined Patent Publication No. H3-81375 (Patent Document 1)). The cuff pressure corresponding to a time point when a maximum amplitude value of a pulse wave is detected is decided as an average blood pressure. According to the oscillometric method, the systolic blood pressure and the diastolic blood pressure are calculated by applying a predetermined algorithym to the cuff pressure and the pulse wave amplitude.
Moreover, a blood pressure measurement method by a volume oscillometric method has also been proposed (“Indirect Measurement of Arterial Pressure Using Volume Pulsation in the Human Finger” by Kenichi Yamakoshi, The Japanese jounal of medical instrumentation published on Nov. 1, 1983, Vol. 53, No. 11, Separate vol. p. 24 to 28 (Non-Patent Document 1)). Specifically, a volume pulse wave sensor is provided in a cuff and in a process in which a cuff pressure is increased (or decreased), a volume pulse wave component (ΔV) is measured and a maximum point of amplitude and a vanishment point (or appearance point) are detected. The cuff pressures corresponding to the respective detected points are calculated as an average blood pressure and a systolic blood pressure. According to the volume oscillometric method, a diastolic blood pressure is calculated by applying the average blood pressure and the systolic blood pressure to a predetermined calculating equation.    Patent Document 1: Japanese Examined Patent Publication No. H3-81375    Non-Patent Document 1: “Indirect Measurement of Arterial Pressure Using Volume Pulsation in the Human Finger” by Kenichi Yamakoshi, The Japanese jounal of medical instrumentation published on Nov. 1, 1983, Vol. 53, No. 11, Separate vol. p. 24 to 28